1. Field of the Invention
The present invention relates to a compound resin wheel, and is particularly concerned with a lightweight and mass-producible compound resin wheel which is surpassing in mechanical characteristics such as impact resistance, bending strength, rigidity, heat resistance, fatigue resistance, creep resistance and the like.
2. Description of the Prior Art
Wheels, or automobile wheels, for example, have ever been made generally of steel or light alloys (such as aluminum alloy and magnesium alloy).
Steel wheels are fabricated by pressing or roll forming, however, a dimensional dispersion is quite unavoidable in most cases, and a deviation is easy to occur particularly on the roundness of a bead seat of the rim. Moreover, the steel wheels are heavy and hence are not preferable for lightweight construction of automobile parts so required.
In the meantime, wheels made of light alloys are stably formed in dimensions and, in addition, sharply reduced in weight to one third of the steel wheels, however, what is disadvantageous is that the material cost is three to five times higher than that of the steel wheels.
Now, the lightweight requirement for the automobile parts is very important from the viewpoint of energy saving on which an emphasis has been placed lately, and thus the wheels must be made lightweight in respect of the fuel cost and the maneuverability.
Under such circumstances, a resin wheel has been proposed for satisfying the lightweight construction and the dimensional stability and the low manufacturing cost. The resin wheel is formed mainly of FRP (fiber-reinforced plastic) obtained from mixing short or long reinforcing fibers in a resin, and hence is light in weight as compared with the metallic wheel, superior in dimensional stability, available for mass production and also for reduction of manufacturing cost, and is excellent, still further, in the aspect of design factors such as coloration and the like.
Meanwhile, almost all of the resin wheels under examination so far and all the resin wheels developed hitherto are those made by the general FRP manufacturing technique, and so formed by pressing a thermosetting resin such as unsaturated polyester, epoxy resin or the like and FRP consisting of glass fiber and others (Japanese Patent Unexamined Publication No. 61-135801). The resin wheels thus obtained are superior in rigidity, strength and so forth, since fibers are not so severely damaged at the time of forming. In addition, the thermosetting resin is used mainly therefor, and hence the resin wheels are also superior in heat resistance and creep resistance. However, such process is not suitable for forming those with a complicated shape (for example, such one as is considerably variable in wall thickness), and a forming cycle is so long, thus leading to an unsatisfactory productivity.
On the other hand, an injection molding may ensure a high productivity and is superior in the aspect of production cost, however, injection moldings are generally 1/3 to 1/5 low in strength as compared with compression moldings. Thus, the injection molding does not satisfy the requirements of strength and rigidity of the products, because the reinforcing, fibers are extremely short in length such as 1 mm or less as compared with materials (such as BMC, SMC and the like) used for compression molding so as to increase the fluidity of the materials.